Debate Continues over Sun's Role in Climate Change

Does the sun's activity affect global warming?

Variations in the Sun’s luminosity, or brightness, since 1978 have been too small to account for the global warming that has occurred in the past 30 years, according to an international team of researchers. Their study, published in the September 15 issue of Nature, found no evidence that changes in total solar irradiance (TSI) are strong enough to have caused climate change on a centennial, millennial, or even million-year timescale. TSI can vary depending on the presence or absence of bright spots (called faculae) and darker sunspots that form or rotate over the Sun’s surface.

Previous research, however, claims that the Earth’s climate is extremely sensitive to even minor changes in the Sun’s energy output. Although the Sun itself cannot be blamed for global warming, some scientists claim that weakened solar output may provide a respite from rising temperatures. Both the Little Ice Age in the second half of the 17th century and the cold period between 1420 and 1570 A.D., which transformed Greenland from fertile farmland to an Arctic icescape, coincided with prolonged lulls in solar activity. According to the U.S. National Aeronautics and Space Administration (NASA), solar energy dropped by three-tenths of one percent and global temperatures fell by about 1 degree Celsius during the Little Ice Age.

Some astronomers now predict a similar drop in solar activity, though not of the same magnitude as the crash between 1645 and 1715. Sunspot activity operates on an 11-year cycle, with major crashes in solar activity occurring approximately every 200 years. The last 50 years have been marked by a particularly restless sun, according to scientists, with large numbers of sunspots forming and disappearing. Nigel Weiss, a solar physicist at the University of Cambridge, says that periods of high solar activity typically do not last longer than 50–100 years, and that a crash is inevitable. Recent calculations by a team at the Max Planck Institute for Solar System Research in Germany suggest that such a crash could result in a 0.2 degree Celsius cooling of the Earth’s atmosphere. If correct, the effects would be on par with the strictest adherence to the Kyoto Protocol’s targets through 2050.

But not everyone agrees that decreased solar activity will have a significant impact on our climate. Although NASA suspects that the Little Ice Age was caused by a prolonged period of lower TSI, it has conceded that greenhouse gases have since superseded the Sun as the dominant factor affecting climate change. Other scientists claim that variations in solar luminosity were not enough to cause the Little Ice Age, or any other climactic changes before or since.

Observations over the next few years should shed light on which school of thought is correct. Yet even if reduced TSI does lower temperatures temporarily, it will not address the real problem facing the Earth’s climatic system—that increases in greenhouse gas concentrations have led to global warming. Swings in solar output have not been large enough to account for the 0.6 degree Celsius increase in global temperatures over the past century. According to Leif Svalgaard, an expert in solar physics, the impending crash in solar activity can at best be considered a temporary reprieve from the warming, perhaps providing enough time for leaders to take real action to curb greenhouse gas emissions. However, Svalgaard warns, if we do nothing, once the solar crash ends, solar activity and rising temperatures will return with a vengeance.

This story was produced by Eye on Earth, a joint project of the Worldwatch Institute and the blue moon fund. View the complete archive of Eye on Earth stories, or contact Staff Writer Alana Herro at aherro [AT] worldwatch [DOT] org with your questions, comments, and story ideas.